1978 Mark Coolant Leak Mystery

Hello Bill,
I hope all is well for you in Lincoln Land.
This might seem like a strange problem to you but here goes. I have some type of a coolant leak. You can leave the car running over a dry surface and no leaks appear. I don’t see where there is any fluid coming off the radiator or hoses. It’s a fairly fast leak and I have to add coolant about every three days if I’m driving it once a day.
There are no head gasket issues and no smoke from the exhaust, except on start up temporarily and it doesn’t smell like coolant and my oil is dark so I know the head gasket is fine, plus if it was leaking that much It would be smoking non stop.
So my big question is where do you think the leak might be coming from and why is there no leak when the engine is running and I’m in park on my driveway???
Sincerely,
Confused Lincoln Lover Justin
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Greetings Justin –
If you are sure that the internal engine seals and gaskets are ok, the only other way to loose coolant is from an external leak that can be found, or the system is being overfilled when the engine is cold and the excess is disappearing out the overflow system as designed. If you are sure that you are in fact not overfilling, then the best method to find an external leak is to have the cooling system pressurized by using the appropriate Cooling System Pressure Pump that most automotive service shops have. This device can be used on a cold engine and a hot engine (some leaks only appear at certain engine temps). The gauge on this device can be observed for a pressure drop at the same time as the leak will appear to the observant technician. The leak should appear in the usual locations such as radiator, radiator cap, coolant hoses, heater core, water valve, water pump, freeze plugs, etc., etc. If the pressure on the gauge does in fact drop thus indicating a leak and no leak can be seen, then you may indeed have an internal engine leak but I wouldn’t lean towards that conclusion until I have fully exhausted all of the other possibilities. Be positive that you are not overfilling the cooling system (there are cold fill markings on the radiator or overflow tank), and that your radiator cap is not defective. The Pressure Pump can also be configured to test the radiator cap.
Some leaks require some time and concentrated automotive detective work, and the location of the leak may surprise you. Please let us know what problem you find so that we may post the information here for others to see.
Good Luck –
Bill